The present invention relates to a superconducting tube for shielding magnetic fields, and, more particularly, to a large superconducting tube for shielding magnetic fields which is suitable for application to an apparatus for measuring magnetic fields of a human body and which assures reduced distortion when laid with its axis in the horizontal direction.
In recent years, research and development has been conducted on oxide superconductors having a high critical temperature, and practical application thereof has thus become realistic. It is well known that superconductors are capable of shielding magnetic fields due to the Meissner effect. Hence attempts have been made to apply oxide superconductors to magnetoencephalongraph (MEG) and nuclear magnetic resonance imaging.
In the application of the oxide superconductor to MEG, usually a large tube is used to accommodate a human body in a lying position inside the tube and to have entirely uniform superconductivity inside the tube without distortion. The oxide superconductor is generally formed on a metal substrate with a sufficient mechanical strength.
In the above-described superconducting tube for shielding magnetic field, a reduction in the entire load thereof is desirable for the ease of handling. This is conventionally achieved by the use of a relatively thin metal plate having a thickness of about 3 mm or less as the metal substrate.
However, since the magnetic-field-shielding superconducting tube is usually used with its axis laid in the horizontal direction, the use of a thin substrate readily deforms the tube in a radial direction thereof, thus generating a distortion in the superconducting layer formed on the substrate. As a result, the superconductivity inside the tube may not be homogeneous.
In order to prevent generation of such distortion, the thickness of the substrate may be increased. However, an increase in the thickness increases the weight of the tube, making handling of the tube difficult. Also, an increase in the thickness increases the distortion in the radial direction of the tube lying with its axis in the horizontal direction due to the weight of the tube, thus increasing the internal stress of the tube and thereby generating cracks in the superconducting layer on the substrate. As a result, the ability for shielding magnetic fields of the superconducting tube greatly deteriorates.